Solid state lighting (SSL) has several advantages compared to conventional lighting, including low power consumption, long lifetime, and small form factor. An important element of SSL is the LED (Light-Emitting Diode) die/chip. The basis for the LED is a semiconductor material that undergoes a complex production process. Several metrology and inspection steps are done during and after the production process of the LED.
Measuring the output power of an LED is typically done using a probing system. In this system, electrical contact is made with each LED die, and a measurement is done of the generated light output power and, optionally, the wavelength.
The international patent application WO98/11425 discloses a method and apparatus for detecting defects in a semiconductor or silicon structure at room temperature and in an efficient time using photoluminescence. The invention employs the use of a high intensity beam of light, preferably having a spot size between 0.1 mm-0.5 μm and a peak or average power density of 104-109 W/cm2, to generate a high concentration of charge carriers, which charge characters detect defects in a semiconductor by interacting with same. These defects are visible by producing a photoluminescence image of the semiconductor. Several wavelengths may be selected to identify defects at a selective depth. Additionally, the method uses confocal optics. This method probes a very small volume of the material with one or more laser beams having very small spot size.
Another method is described in U.S. Pat. No. 7,504,642 B2, wherein one or more images are created using filtering and image computation to selectively create a defect image of one selected layer of a wafer, while trying to eliminate unwanted contributions of other layers of the same wafer. The method uses photoluminescence to identify defects in one or more specified material layers of a sample. One or more filtering elements are used to filter out predetermined wavelengths of return light emitted from a sample. The predetermined wavelengths are selected such that only return light emitted from one or more specified material layers of the sample is detected. Additionally or alternatively, the wavelength of incident light directed into the sample may be selected to penetrate the sample to a given depth, or to excite only one or more selected material layers in the sample. Accordingly, defect data characteristic of primarily only the one or more specified material layers is generated.
The international patent application WO 2007/128060 A1 describes a method for testing of indirect bandgap (e.g. Si) semiconductor materials with photoluminescence, based on a comparison of several regions in two or more images. The method is suitable for identifying or determining spatially resolved properties in indirect bandgap semiconductor devices such as solar cells. In one embodiment, spatially resolved properties of an indirect bandgap semiconductor device are determined by externally exciting the indirect bandgap semiconductor device to cause the indirect bandgap semiconductor device to emit luminescence, capturing images of the luminescence emitted from the indirect bandgap semiconductor device in response to the external excitation, and determining the spatially resolved properties of the indirect bandgap semiconductor device based on a comparison of the relative intensities of regions in one or more of the luminescence images.
Quality control of LEDs is becoming more and more crucial since LEDs are used for illumination. For example, it is important that LEDs used for the back illumination of a TV set are of similar intensities. Therefore, quality control of the light output power of LEDs is needed. Previously, such quality control involved electrically contacting the LED (probing) and measuring the emitted light output power. This has several disadvantages: LEDs may get damaged during probing, probing is slow, and probing requires an additional tool.